Significant Hall–Petch effect in micro-nanocrystalline electroplated copper controlled by SPS concentration

Electroplated Cu has been extensively applied in advanced electronic packaging, and its mechanical properties are critical for reliability. In this study, Cu foils fabricated through electroplating with various bis-(3-sulfopropyl) disulfide (SPS) concentrations are examined using tensile tests. The SPS concentration affects the grain size of the electroplated Cu foils, resulting in different mechanical properties. A significant Hall–Petch effect, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{y} = 197.4 + 0.12{d}^{\frac{-1}{2}}$$\end{document}σy=197.4+0.12d-12, is demonstrated for the electroplated Cu foils. The different concentrations of impurities identified through time-of-flight secondary ion mass spectrometry correspond to the different grain sizes, determining the transgranular and intergranular fracture during the tensile test. The results demonstrate that the SPS concentration controlling the microstructures of the electroplated Cu results in a Hall–Petch effect on the mechanical properties of the electroplated Cu foils.

In the past, aluminum was used as the primary interconnecting material in electronic packaging; however, the high demand for interconnecting materials with the development of advanced electronic packaging has led to the replacement of aluminum with copper (Cu). This is because Cu exhibits better electrical conductivity and resistance to electromigration than aluminum. Additionally, the excellent thermal conductivity, ductility, relatively high melting temperature, and appropriate strength of Cu have made it a popular conductor material in electronic products 1,2 .
Electroplating of Cu is important for industrial mass production in the fabrication of conductive traces, wires, and metallization in electronic devices [3][4][5] . Currently, most electroplating solutions for semiconductor and printed-circuit board factories are commonly composed of sulfuric acid and copper sulfate because of their low toxicity and excellent management of the plating baths [5][6][7] . In contrast, organic additives added to the electroplating solutions are vital in controlling the deposition rate of reduced Cu atoms and microstructures of the electroplated Cu. For instance, some additives in the plating solutions can be used to fabricate Cu films with nanotwin structures to enhance their electricity, strength, and void suppression 5,8,9 . One of the additives is the chloride ion (Cl − ) from NaCl or HCl, which increases the reduction rate of Cu ions 10 . In addition, Cl − can cowork with other additives, such as polyethylene glycol (PEG), to suppress the rate of Cu reduction on the cathode surface 11,12 . Bis-(3-sulfopropyl) disulfide (SPS) reacts with Cl − to accelerate the reduction rate of Cu ions on the cathode surface and reduce the surface roughness of the electroplated Cu 13 . The variation in the concentrations of the additives significantly affected the microstructures of the electroplated Cu because of the change in the deposition kinetics of the reduced Cu atoms 14 . Therefore, the influence of the concentration of the additives on the properties of electroplated Cu is worth investigating.
In recent years, three-dimensional integrated circuits have become an essential solution for fabricating highperformance electronic products with extreme miniaturization 15,16 . Electroplated Cu has been widely applied in redistribution layers (RDLs) and through-silicon vias (TSVs) in advanced electronic packaging such as fan-out wafer-level packaging 17,18 . In RDLs and TSVs, the Cu wires must pass through silicon wafers and polymer substrates (epoxy molding compound). The latter exhibits a high thermal expansion, whereas the thermal expansion of the former is very low, and that of Cu ranges between them. Thermal stress is generated in the Cu wires by

Experimental procedures
Electroplating Cu film. A glass plate affixing a Cu foil (Alfa Aesar, 99.8% purity, 25 μm thick) and an acid-resistant tape with a dog-bone shaped space area was used as the substrate for the electroplating of Cu at the cathode of the electroplating bath (Fig. 1a). The anode of the electroplating bath was a Cu-0.04 wt% P plate cleaned using sulfuric acid (2 vol%) and dilute hydrogen peroxide. The electrolyte mainly consisted of highpurity CuSO 4 ·5H 2 O and 5 vol. % H 2 SO 4 (purity: 95-98%). The electroplating solutions comprised the electrolyte, 60 ppm Cl − , 50 ppm PEG, and 0-2.0 ppm SPS for fabricating the electroplated dog-bone shaped Cu films. A potentiostat (CHI-611E, CH Instruments, Austin, USA) controlled the direct current under a current density of 4 ASD, and a magnetic stirrer provided mechanical stirring at 1000 rpm to fabricate a uniform electroplated Cu, as shown in Fig. 1b. According to the electroplating rates, the thickness of the Cu films was set at approximately 50 μm.
Tensile test. After electroplating, the electroplated Cu samples were carefully removed from the substrates.   www.nature.com/scientificreports/ Figure 3a shows the top-view optical images of the electroplated Cu foils peeled from the glass substrate after electroplating with SPS concentrations of 0, 0.2, 0.5, 1.0, and 2.0 ppm. The specimens were labelled as PC, PCS0.2, PCS0.5, PCS1.0, and PCS2.0, respectively. Although the top-view morphology of PCS0.2 is very similar to that of PC, the images show that the surface brightness of the Cu foil was significantly enhanced by increasing the SPS concentration. This is because the increase in the concentration of SPS gradually replaced the PEG molecules (suppressor) attached to the electroplated surface, accelerating the reduction of Cu ions 13,14 . When the concentration of SPS was low (0.2 ppm), the effect of the accelerator on the electroplating was very limited; therefore, the morphologies of PC and PCS0.2 resembled each other. When the concentration of SPS was increased to 0.5 ppm, the SPS molecules began to affect the Cu reduction. An increase in Cu reduction provided a uniform electroplating rate on the electroplated surface at the cathode to lower the roughness of the electroplated Cu surface. The SPS was also referred to as a brightener, and the Cu foils of PCS0.5-2.0 were brighter than those of PC and PCS0.2. The effect of SPS on the roughness of the electroplated Cu foil is illustrated by the SEM images in Fig. 3b. The top-view morphology of PC was very rough and had large cone structures, and the size of the cones was significantly reduced by 0.2 ppm SPS. Furthermore, the cones mostly disappeared when the concentration of SPS was ≥ 0.5 ppm, with the electroplated surface being very smooth. Excellent surficial uniformities of PCS0.5-2.0 were be observed in the higher-magnification SEM images (× 10,000), as shown in Fig. S1. Although the rough surface could be improved through an electropolishing process following electroplating 21    www.nature.com/scientificreports/ with smooth surfaces (PCS0.5-2.0), and PCS0.2 exhibited the highest tensile strength. Conversely, the elongation of the latter was better than that of the former. Table 1 summarizes the average yield stress and elongation of the five tensile specimens for each electroplating condition. The average yield stress of PCS0.2 was the highest (416 MPa), and the average elongation of PCS2.0 was the highest while that of PCS1.0 was very close to it. The PCS0.5 exhibits intermediate yield strength and elongation among the foils. The trend of these results primarily corresponds to that of the stress-strain curves shown in Fig. 4. In a polycrystalline metal, the strengthening mechanism is mainly attributed to the crystalline size and concentration of impurities 24 . The SEM images shown in Fig. 3b indicate a decrease in the size of the cones when 0.2 ppm SPS was added to the electroplating solution of PC. If the cones can be regarded as multiple grains, the grain size difference in the Cu foils can be changed by the addition of SPS. Thus, grain size reduction is one of the main reasons to reinforce the foils 25 . This is because metallic deformation requires the movement of the dislocations in the metal, and the grain boundary is an obstacle that blocks the dislocation movement. If there were more grain boundaries, that is, a smaller grain size to significantly stop the dislocation slips, the strength of the deformed metal was enhanced, and this phenomenon was referred to as the Hall-Petch effect 26 . The Hall-Petch equation can be expressed as follows:

Results and discussion
where σ y denotes the yield strength that varies with the grain size, σ y,0 is the original yield stress, k is a constant, and d is the grain size 27 . Moreover, EBSD can precisely analyze the average grain size of electroplated Cu foils 28 . Figure 5 shows the EBSD mapping of the grains in the electroplated Cu foil. The grains in PC were small but slightly larger than those in PCS0.2, and the grain size increased with increasing SPS concentration.  www.nature.com/scientificreports/ among all the foils and the highest strength. When the grain size was on the nanoscale, the dislocation slips quickly encountered grain boundaries, inducing the Cu foils of PC and PCS0.2 to reinforce and deform. Thus, they exhibited high strengths and low elongations 29 . Conversely, the grain sizes of PCS1.0 and PCS2.0, which were ten times larger than those of PCS0.2, exhibited higher elongations. Additionally, PCS0.5, with an intermediate grain size, exhibited medium strength and elongation. Figure 6 illustrates the yield stress data with the inverse square root of the grain size. The linear fitting for the data points was σ y = 197.4 + 0.12d −1 2 , whose tendency certainly met the Hall-Petch effect. The constant k of the electroplated Cu in this study was 0.12 MPa m 1/2 , which is close to that obtained in a previous study (0.14 MPa m 1/2 ) 30 . Boundary strengthening was demonstrated by the mechanical properties of the electroplated Cu foils. In contrast, the X-ray diffraction patterns in Fig. S2 exhibit that the grain orientations were distributed randomly in those Cu foils. The effect of Cu grain orientation on the mechanical properties of the Cu foils can be ignored, and the Hall-Petch equation was very suitable for evaluating the strengths of the foils.
The difference in grain size can be attributed to the impurities in the electroplated Cu foils. Impurities originating from the electrolytes and additives of the plating solutions were inevitably co-deposited with the reduced Cu atoms and were likely to exist at the crystalline boundaries in the as-electroplated Cu 5,9 . The impurities at the boundaries suppressed the growth of the electroplated Cu crystals; that is, the impurities pinned the movement of the boundaries during crystal growth owing to the drag effect. Moreover, the process occurred at room temperature and did not provide sufficient kinetic energy to migrate the grain boundaries blocked by the impurities. Therefore, if there are numerous impurities in the electroplated Cu, the grain size is typically small 31 . This inference is supported by SIMS analysis of chloride, carbon, sulfur, and oxygen in the electroplated Cu foils of PC, PCS0.2, PCS0.5, PCS1.0, and PCS2.0, as shown in Fig. 7. The PC and PCS0.2 samples with smaller grain sizes incorporated more impurities than PCS0.5, PCS1.0, and PCS 2.0, which had larger grain sizes. In particular, PCS0.2, which had the smallest grain size, contained the highest intensities of C and O. The results demonstrated the effect of impurities on the Cu grain size and mechanical properties of the electroplated Cu. Figure 8 shows the fracture surfaces of PC, PCS0.2, PCS0.5, PCS1.0, and PCS2.0 after the tensile test. Several dimple structures were observed on the fracture surfaces of PC and PCS0.2 Cu. The dimple structures surrounded by grains were morphological after an intergranular fracture. When impurities accumulated at the grain boundaries in the Cu foils, the grain boundaries became significantly weak points for stress concentration. Consequently, we observed intergranular fractures in Cu foils with a high concentration of impurities. In contrast, PCS0.5-2.0 Cu with large grain sizes contained a significantly low concentration of impurities, and the fracture surfaces with the extension of grain boundaries owing to tensile stress exhibited a transgranular fracture mode 32 . The fracture modes correspond to the SIMS analysis.

Conclusions
In this study, we examined the mechanical properties of electroplated Cu foils with SPS concentrations ranging from 0 to 2.0 ppm. The top-view optical images illustrated that an increase in SPS improved the brightness owing to the improvement in the roughness of the electroplated surface. In the tensile tests, SPS0.2 Cu exhibited the highest yield strength, whereas SPS1.0 and 2.0 exhibited significant elongations. According to the EBSD analysis, the grain size in the latter was ~ 10 times larger than that in the former. The Hall-Petch effect on the mechanical properties of the electroplated Cu was significant and it obeyed the linear fitting of σ y = 197.4 + 0.12d −1 2 . The small grain size of electroplated Cu was attributed to the high concentration of impurities identified through SIMS. When more impurities were present at the grain boundaries, the Cu foils grain size was small. Impurities at the grain boundaries affected not only the grain size but also the fracture mode in the tensile foils. Cu foils with low and high concentrations of impurities were broken by transgranular and intergranular fractures, respectively. The obtained results demonstrated that the SPS concentration controlled the microstructures of the electroplated Cu, resulting in a significant Hall-Petch effect on the mechanical properties.

Data availability
The datasets used and analyzed in this study are available from the corresponding author upon reasonable request.